Generally, the top wiring layer of a multilayer wiring using an aluminum alloy in semiconductor devices has a structure as shown in FIG. 9B. In a conventional method shown in FIG. 9, on an insulating film 10 is formed a high melting metal layer 12 having TiN, Ti, or the like as a main component of about 30 to 100 nm, and an aluminum alloy layer 14 of a desired thickness is formed thereon by sputtering, as shown in FIG. 9A. Then, on the aluminum alloy layer 14 is formed an anti-reflection film 16 having TiN as a main component.
Next, after transferring a pattern by a photolithography technique, patterning is performed on the layered structure formed as shown in FIG. 9A by dry etching using a Cl based gas as shown in FIG. 9B. In this condition, a resist (not shown) is removed by O2 ashing at about 300° C. Next, annealing is performed at about 350° C. to 400° C. in a gas containing H2. Then, as shown in FIG. 9C, a SiN film 20 is formed as a passivation film by a CVD technique.
Japanese Unexamined Patent Publication No. 2003-243570 discloses an invention related to an inductor using a thick aluminum wiring.
Normally, the thickness of an aluminum alloy film used for wiring is 1 μm or less. However, recently, in some cases in high frequency devices, aluminum alloy films having a thickness of about 2.5 to 5.0 μm are used. Conventionally, since the design rules are not strict, in many cases, wet etching using an acid or the like is performed on wirings having such a thickness.
However, recently, due to reasons of design rules, the number of cases are being increased in which aluminum wirings having such a thickness have to be worked by dry etching. In such cases, problems may occur in which, after performing heat treatment such as ashing or sintering after etching the wiring, a stress in the aluminum itself causes troubles such as fluctuation of characteristics of the base transistor and cracking of the interlayer insulating film just below the aluminum layer. When a heat history is applied to the aluminum alloy (wiring layer) in a step after formation, since the coefficient of thermal expansion is different, a compressive stress is applied to the aluminum when the temperature rises, causing plastic deformation. The stress is relieved at the same time as when the aluminum is plastically deformed, and the crystals of aluminum are more stabilized due to grain growth and elimination of defects. Cooling thereafter provides a large tensile stress.
In normal wirings (thin wirings), there are only a few cases where the stress itself becomes problematic. However, if the aluminum alloy becomes 5 to 10 times thicker than a normal wiring, the total stress is increased for that amount, and the effect on the base is also increased. Generally, the stress can be relieved by making slits in the aluminum wiring itself. However, in terms of the performance of high frequency elements, introduction of slits is difficult in many cases. Moreover, in thick aluminum wirings, the slit release property by etching is not satisfactory, and the slit should be wider than 1 μm, thus making slits itself difficult.
FIG. 4 shows a thermal stress curve of an aluminum layered film having a thickness of 400 nm which is typically used for wirings. Since aluminum has a greater coefficient of thermal expansion than that of a semiconductor substrate, a compressive stress occurs in the aluminum film as it is heated. Moreover, plastic deformation occurs, for example, during a recrystallization process, at about 250 to 300° C. Then, the stress is relieved as the aluminum recovers from defects, the grains grow, and hillocks partially occur.
FIG. 5 shows a thermal stress curve of an aluminum layered film having a thickness of 3000 nm. It is shown that, in a thick aluminum layer, plastic deformation starts at a low temperature of about 100 to 150° C. Since the stress is generally shown by a value normalized by the film thickness, the value of the stress looks small in the graph. However, as the film gets thicker, the total stress is increased for that amount. If the thickness of the aluminum film is 4000 nm (4 μm), the total stress becomes 10 times. A thick film has a large stress of its own, and thus yields at a lower temperature. Moreover, in a thick aluminum film, since grains grow largely in the process of film formation, the film is prone to have less defects therein and thus be readily plastically deformed. This is considered to be the reason why the film readily yields at an even lower temperature.